Processes such as catalytic cracking operations are commercially employed in the petroleum refining industry to produce gasoline and fuel oils from hydrocarbon-containing feeds. These cracking operations also result in the production of useful lower olefins, e.g., C3-C5 olefins, and it has become increasingly desirable to maximize the yield of such olefins from conversion process operations in general. Endothermic catalytic cracking of hydrocarbons is commonly practiced in fluid catalytic cracking (FCC) processes.
Generally, FCC is commercially practiced in a cyclic mode. During these operations, the hydrocarbon feedstock is contacted with hot, active, solid particulate catalyst without added hydrogen, for example, at pressures of up to about 50 psig and temperatures up to about 650° C. The catalyst is a powder with particle sizes of about 20-200 microns in diameter and with an average size of approximately 60-100 microns. The powder is propelled upwardly through a riser reaction zone, fluidized and thoroughly mixed with the hydrocarbon feed. The hydrocarbon feed is cracked at the aforementioned high temperatures by the catalyst and separated into various hydrocarbon products. As the hydrocarbon feed is cracked in the presence of cracking catalyst to form gasoline and olefins, undesirable carbonaceous residue known as “coke” is deposited on the catalyst. The spent catalyst contains coke as well as metals that are present in the feedstock. Catalysts for FCC are typically large pore aluminosilicate compositions, including faujasite or zeolite Y.
The coked catalyst particles are separated from the cracked hydrocarbon products, and after stripping, are transferred into a regenerator where the coke is burned off to regenerate the catalyst. The regenerated catalyst then flows downwardly from the regenerator to the base of the riser.
These cycles of cracking and regeneration at high flow rates and temperatures have a tendency to physically break down the catalyst into even smaller particles called “fines”. These fines have a diameter of up to 20 microns as compared to the average diameter of the catalyst particle of about 60 to about 100 microns. In determining the unit retention of catalysts, and accordingly their cost efficiency, attrition resistance is a key parameter. While the initial size of the particles can be controlled by controlling the initial spray drying of the catalyst, if the attrition resistance is poor, the catalytic cracking unit may produce a large amount of the 0-20 micron fines which should not be released into the atmosphere. Commercial catalytic cracking units include cyclones and electrostatic precipitators to prevent fines from becoming airborne. Those skilled in the art also appreciate that excessive generation of catalyst fines increases the cost of catalyst to the refiner. Excess fines can cause increased addition of catalyst and dilution of catalytically viable particles.
Additionally, the catalyst particles cannot be too large in diameter, or the particles may not be sufficiently fluidized. Therefore, the catalysts are preferably maintained under 120 to 150 microns in diameter.
Particulated catalyst additives are also typically included in the inventory of conventional large pore cracking catalysts for FCC processes and are therefore subject to the same attrition issues. These additives are very useful in enhancing the properties of the resulting gasoline product as well as enhancing octane numbers of the gasoline product. Such additives also are especially suitable for enhancing yields of C3-C5 olefins. Those olefins are useful in making ethers and alkylates which are in high demand as octane enhances for gasoline, as well as useful in making other chemical feedstocks.
Particulated catalysts and additives are prepared from a number of compounds in addition to the primary active catalytic species. For example, the catalyst compositions can comprise clay and other inorganic oxides in addition to catalytically active ZSM-5. Alumina is one particular inorganic oxide other than zeolite that can be added. EP 256 875 reports that alumina in conjunction with rare earth compounds improves hydrothermal stability and selectivity of zeolite Y. Phosphorous also is added to “stabilize” ZSM-5 containing catalysts. Additives sold as OlefinsMax™ by Grace Davison is an example. Stabilization of a catalyst composition means stabilizing the activity of the composition to produce higher yields of light olefins when compared to a composition which has not been stabilized by phosphorus. This comparison is normally made after deactivation with steam.
U.S. Pat. No. 5,110,776 teaches a method for preparing FCC catalyst comprising modifying the zeolite, e.g., ZSM-5, with phosphorus. U.S. Pat. No. 5,126,298 teaches manufacture of an FCC catalyst comprising zeolite, e.g., ZSM-5, clay, and phosphorus. See also WO 98/41595 and U.S. Pat No. 5,366,948. Phosphorus treatment has been used on faujasite-based cracking catalysts for metals passivation (see U.S. Pat. Nos. 4,970,183 and 4,430,199); reducing coke make (see U.S. Pat. Nos. 4,567,152; 4,584,091; and 5,082,815); increasing activity (see U.S. Pat. Nos. 4,454,241 and 4,498,975); increasing gasoline selectivity (See U.S. Pat. No. 4,970,183); and increasing steam stability (see U.S. Pat. Nos. 4,765,884 and 4,873,211).
In U.S. Pat. No. 3,758,403, use of large-pore cracking catalyst with large amounts of ZSM-5 additive gives only modest increase in light olefin production. A 100% increase in ZSM-5 content (from 5 wt. % ZSM-5 to 10 wt. % ZSM-5) increased the propylene yield less than 20%, and decreased slightly the potential gasoline yield (C5+ gasoline plus alkylate).
When attempting to improve or enhance the catalytic activity of these compositions, the amounts of the various components in a catalyst or catalyst additive and the relevant effect these components have on attrition have to be taken into account in order to maximize attrition resistance. The importance of attrition becomes increasingly acute when, for example, the ZSM-5 content of a catalyst is increased to enhance the catalyst's activity. In certain instances, increasing a catalyst's ZSM-5 content results in the use of less binder and matrix, and as a result, “softer” or more attrition prone particles can be created. Even though particles having a ZSM-5 content up to 60% and an attrition index less than 20 have been reported (U.S. Pat. No. 5,366,948), it has been difficult to prepare catalysts and additives which contain a great majority, i.e., greater than 60% of the active component over the other components in the catalyst. For example, it would be desirable to increase the amount of ZSM-5 to these high levels in certain catalysts in order to produce a particle which is more active in producing C3-C5 olefin.
Refiners, e.g., FCC refiners, DCC (Deep Catalytic Cracking) refiners, as well as fixed fluidized bed refiners, would also find it advantageous to enhance ethylene yields in order to maximize the yield of valuable products from their refinery operations. Additives or compositions comprising novel catalysts are potential avenues for enhancing ethylene yields. Using those additives or compositions, however, without materially affecting the yield of other olefins can be difficult, especially in light of the other concerns mentioned above with respect to attrition.
Therefore, with certain refiners, it would not only be highly desirable to prepare a catalyst composition having a high attrition resistance, it would also be desirable to provide catalyst compositions having improved activity for ethylene production as well as substantially maintain the compositions' ability to produce other olefins. Those skilled in the art will also appreciate that improved attrition resistance as well as improved activity will translate into reduced catalyst makeup rates.
Attrition resistance and high catalyst content would also benefit processes used to react hydrocarbons other than hydrocarbon cracking processes. Such processes include hydrocarbon isomerization, dimerization and the like.